The invention relates to an evaluation unit of a differential-pressure sensor having
a sensor element with PA1 a temperature sensor which is arranged on the sensor element and PA1 a temperature measuring circuit which is connected to the temperature sensor and which supplies an output signal which corresponds to the sensor temperature, which evaluation unit comprises: PA1 a measuring circuit for determining the respective reciprocal values of the capacitances of the measuring capacitors and PA1 an arithmetic unit PA1 a sensor element with PA1 a temperature sensor which is arranged on the sensor element and PA1 a temperature measuring circuit which is connected to the temperature sensor and which supplies an output signal which corresponds to the sensor temperature, which evaluation unit comprises: PA1 a measuring circuit for determining the respective reciprocal values of the capacitances of the measuring capacitors and PA1 an arithmetic unit PA1 an internal temperature T.sub.i is assigned to the sum of the reciprocal values of the capacitances, PA1 a first correction value is determined in accordance with EQU .DELTA.K=T.sub.i -T.sub.s -T.sub.0, PA1 a differential pressure which has been corrected with respect to a measuring error due to the static pressure is determined from the differential-pressure measured value using the first correction value. PA1 the reference correction value being the first correction value recorded at a maximum permissible static pressure, at a differential pressure of 0 Pa and at a reference temperature and PA1 the zero-point discrepancy being the difference between a differential pressure measured at the reference temperature and at the maximum static pressure and a differential pressure likewise measured at the reference temperature, but at a static pressure equal to zero, PA1 S being a desired span, that is to say the measuring range span which the differential-pressure sensor is to have, and PA1 I being an actual span, that is to say the span which the differential-pressure sensor actually has at the reference temperature and at the maximum permissible static pressure, and PA1 .DELTA.K.sub.R being the reference correction value which is equal to the first correction value .DELTA.K determined at the reference temperature T.sub.R, at the maximum permissible static pressure P.sub.Smax and at a differential pressure of 0 Pa.
a liquid-filled measuring chamber, PA2 a first diaphragm to which a first process pressure which acts on the sensor element is applied, the first process pressure corresponds to the sum of a static pressure and a first pressure, PA2 a second diaphragm to which a second process pressure which acts on the sensor element is applied, which the second process corresponds to the sum of the static pressure and a second pressure, and PA2 two measuring capacitors whose capacitances change in the opposite direction to the difference between the first and second pressures and in the same direction as the static pressure, PA2 which determines the sum of the reciprocal values of the two capacitances, PA2 which determines the difference between the reciprocal values of the two capacitances and assigns a differential-pressure measured value to said difference, and PA2 which outputs an output signal which corresponds to a differential pressure, specifically to the difference between the first and second pressures. PA2 a liquid-filled measuring chamber, PA2 a first diaphragm to which a first process pressure which acts on the sensor element is applied, the first process pressure corresponds to the sum of a static pressure and a first pressure, PA2 a second diaphragm to which a second process pressure which acts on the sensor element is applied, the second pressure corresponds to the sum of the static pressure and a second pressure, and PA2 two measuring capacitors whose capacitances change in the opposite direction to the difference between the first and second pressures and in the same direction as the static pressure, PA2 which determines the sum of the reciprocal values of the two capacitances, PA2 which determines the difference between the reciprocal values of the two capacitances and assigns a differential-pressure measured value to said difference, and PA2 which outputs an output signal which corresponds to a differential pressure, specifically to the difference between the first and second pressures. PA2 the internal temperature T.sub.i being equal, with the exception of a temperature discrepancy, to the sensor temperature T.sub.S, when PA2 T.sub.0 being the temperature discrepancy, determined at the reference temperature, between the sensor temperature T.sub.S and the internal temperature T.sub.i, and PA2 a differential pressure of 0 Pa being applied to the differential-pressure sensor in order to determine the two differential pressures.
DE-C 39 32 443 describes an evaluation unit of a differential-pressure sensor having
In differential-pressure sensors with liquid-filled measuring chambers, an increase in temperature causes the filling liquid to expand. Both diaphragms are deflected outward. Consequently, the values of the two capacitances decrease. The sum of the reciprocal values of the two capacitances is thus a measure of the temperature and, in the case of the subject matter of DE 39 32 443, it is used to correct the differential-pressure measured value and/or measuring error due to temperature.
In addition, the sensor temperature is determined by means of the temperature sensor and the temperature-measuring circuit. If the deviation between the sensor temperature and a temperature which is to be expected on the basis of the sum of the capacitances exceeds a specified limit value, an error message is triggered.
In addition to the aforesaid measuring error due to temperature, a further measuring error, which is described below as the rated pressure error, occurs. Said further measuring error arises as a result of the fact that the static pressure causes the sensor element to be compressed in the axial direction, i.e. in the direction of the surface normal to the diaphragms, and to be correspondingly extended in the radial direction. This leads to a radial extension and thus to an increase in the rigidity of the diaphragms.
The deflection of the diaphragms rises essentially linearly with the differential pressure applied to the sensor element. This differential pressure-dependent deflection decreases essentially linearly as the static pressure rises. The static pressure always brings about an increase in the values of the two capacitances.
When the differential pressure acting on the sensor element is 0 Pa, the rated pressure error is proportional to the static pressure and arises as a result of asymmetries in the structure of the sensor element. This rated pressure error can therefore assume either positive or negative values. In contrast, the rated pressure error in the case of a differential pressure which is different from zero is always negative, i.e. an excessively low differential pressure is measured.
Both causes of errors, static pressure and temperature, affect the same measurement variables, specifically the values of the two capacitances, but must be treated differently when correcting the differential-pressure measured value.
In order to compensate the rated pressure error, it is therefore customary to use an additional sensor, by means of which the static pressure is determined. This sensor is to be mounted at a point at which it is actually subjected only to the static pressure and its measurement result is to be made available to an evaluation unit of the differential-pressure sensor.
An object of the invention is to specify an evaluation unit of a differential-pressure sensor, which unit generates an output signal which corresponds to a differential pressure which has been corrected in terms of its rated pressure error.
For this purpose, the invention consists in an evaluation unit which is distinguished by the fact that
there is no static pressure applied to the two diaphragms and PA3 the difference between the first and second pressures is equal to zero,
which is a measure of the static pressure at a reference temperature,
According to one development of the invention, a second correction value is determined which is equal to the product of the first correction value and a polynomial of the sensor temperature, the polynomial having constant coefficients which are stored in a memory and which are determined in such a way that at a sensor temperature and a specific static pressure, the second correction value assumes in each case the value which the first correction value would assume at the reference temperature if the same static pressure were acting on the differential-pressure sensor.
According to one refinement, the differential-pressure measured value is corrected in terms of a measuring error due to temperature.
According to a further refinement, a differential pressure which has been corrected with respect to a displacement, due to the applied static pressure, of the zero point is determined from the differential-pressure measured value in accordance with EQU .DELTA.P.sub.K =.DELTA.P.sub.M -.alpha..DELTA.Y,
.DELTA.Y being a correction value which is equal to the first correction value when the differential-pressure sensor is used exclusively at temperatures which are equal to the reference temperature or deviate from it only insignificantly, and which is equal to the second correction value when the differential-pressure sensor is not used exclusively at temperatures which are equal to the reference temperature or deviate from it only insignificantly, and a being a constant zero-point correction factor stored in a memory.
According to a further refinement, the zero-point correction factor is equal to the quotient of a zero-point discrepancy and a reference correction value,
According to a further refinement, the differential pressure is determined from the corrected differential-pressure measured value in such a way that it is corrected with respect to a change in the span due to the applied static pressure, in accordance with EQU .DELTA.P=(1+.beta..DELTA.Y).DELTA.P.sub.K,
.beta. being a constant span correction factor stored in a memory.
According to a further refinement, the span correction factor is a characteristic variable of the differential-pressure sensor which is determined in accordance with the equation EQU .beta.=(S/I-1)1/.DELTA.K.sub.R,
In this way, an evaluation unit is specified which permits the rated pressure error to be compensated without the value of the static pressure having to be made available by an additional sensor.
The invention and advantages thereof are explained in more detail with reference to the figure of the drawing, in which an exemplary embodiment is illustrated.